In recent years the entire transportation and shipping industry has undergone a revolution. In an effort to improve delivery times and reduce costs, the shipping industry has become containerized. Instead of moving goods by handling individual cartons and boxes, the modern approach has been to package boxes and cartons into larger standardized "containers". A whole new class of ship has been developed to handle these containers. The containers are transported or moved from one place to another by a spreader, (i.e. a structural frame that conformably engages and attaches to the container). The spreader itself is supported at each end by a plurality of cable and sheave arrangements for raising and lowering the containers. Containers are usually twenty or thirty feet in length and have specially adapted brackets at the corners to receive latches carried by the spreader to support the containers during transportation.
A straddle carrier is one material handling machine which has been developed to transport containers. A straddle carrier generally consists of an inverted U-shaped frame that has wheels on the lower ends of the respective legs. The legs define an elongated open bay with a spreader unit supported inside the bay. The spreader defines the longitudinal axis of the carrier; the spreader normally has latches at the respective corners thereof for coupling to a container. A hoisting means or crane is then used to raise and lower the spreader.
The cranes that support spreaders conventionally include a bridge structure of spaced horizontal girders. These girders extend transversely between the two legs forming the bay. The bridge structure or girders themselves typically carry one or more sets of trolleys.
Two trolleys are often used to spread apart the suspension cables while it is being raised or lowered and to inhibit and arrest swaying of the load. The "skew" of the spreader is a measure of the annular displacement of the spreader about a vertical axis. Two other measures of the alignment of the spreader are "list" and "trim". List and trim are angular displacements measured about horizontal axes. The list and trim axis are mutually perpendicular to each other and perpendicular to the vertical axis defining the skew of the spreader. Since the spreader is supported by two trolleys and since each trolley is independently suspended or carried by a separate girder on the bridge structure, the spreader will be skewed or rotated about a vertical axis unless the trolleys are displaced equally to one side of a vertical plane passing through the trim axis.
Because of the large size of straddle carriers, it is generally difficult for the crane operator to affect the desired alignment of the cable suspended spreader with the cargo container, particularly when the spreader is relatively distant from that operator. Frequently, an additional workman or assistant is necessary to properly align the spreader. Unless the spreader is properly aligned the crane operator will have difficulty joining the spreader to the container.
Heretofore relatively complicated electromechanical and electrohydraulic devices have been used to synchronize and align the two trolleys relative to one another so as to control the amount of skew. The skew control taught by Smith (U.S. Pat. No. 3,204,577) by White et al. (U.S. Pat. No. 3,656,795) and by S. V. Lynd (U.S. Pat. No. 3,166,023) are typical examples.
It can be appreciated from the description given above that unless the operator of the straddle carrier is given some indication of the amount of skew of the spreader, any attempt to join the spreader with the container has the potential for failure. If the productivity of the crane operator and the efficiency of the crane are to be maximized, the operator must be made aware when the spreader becomes skewed beyond a certain point after which there is a marginal probability that the spreader can be successfully coupled with the container unless the trolleys are aligned.